Polymeric coatings and composites are commonly used to protect a substrate from wear and environmental exposure. They can be seen in articles of manufacture in the defense, construction, automobile, aerospace and petroleum industries, just to name a few of them. They are particularly useful where high strength, stiffness, low weight, and environmental stability are required. The coatings can be primers, mid-coats or top-coats. Modern coatings are often formulated to include specialized functional or multifunctional agents to improve or provide various different properties of the coating (e.g., indicators of damage, hydrophobicity, ice repellence, anti-fouling, anti-dusting, anti-corrosion, thermo-protection, self-healing, etc.), and are capable of adapting their properties dynamically to an external stimulus. When a functionalized coating is able to sense the environment and make a desired response to the stimulus, the general term smart coating is often assigned to it.
The integrity of a coating can be compromised by many things, such as, fatigue, impact or scratch damage, which can expose the underlying substrate to a corrosive environment. Corrosion reduces the mechanical performance of substrates, which results in timely and expensive repairs. Polymeric coatings are often used to protect a substrate from corrosion damage. Polymeric coatings are susceptible to damage in the form of small cracks, which can be difficult to detect. Even on a small scale, crack damage can significantly compromise the integrity and functionality of polymeric materials. On metal substrates with polymer coatings, corrosion and other forms of environmental degradation will generally initiate at the damage site, compromising the underlying substrate materials. In fiber reinforced polymer composites, small impact damage that is difficult to detect can lead to significant degradation in mechanical performance.
In view of the advances and increased uses of smart coatings, the importance of damage indication has garnered increased importance. Numerous approaches have been studied to indicate damage in polymeric coatings and biomolecules. A wide range of mechanisms for mechanically triggered color change and fluorescence in polymers have been reported, including single molecule turn-on/off fluorescence, mechanochemistry, phase/morphology/defect evolution and embedded capsules. Single molecule optical techniques enable force detection at small length scales, such as studies of cell adhesion and interfaces in biological systems. Biomolecules have also been exploited as mechanophores to reveal microscopic damage in bulk polymeric composites. However, these detection mechanisms are currently restricted to material interfaces and long-term stability is unknown.
Mechanochemically induced color/fluorescence changes have been generated under large strains in bulk polymers. Many of these early mechanophores exhibit a reversible optical change, and therefore, are not promising to detect permanent damage. A few mechanochemical systems have been developed to indicate damage, but performance has been limited by low intensity and potential bleaching of fluorescence. U.S. Pat. No. 8,846,404 describes a self-indicating polymeric coating where damage-induced rupturing of microcapsules initiates a reaction between a charge-transfer donor and a charge-transfer acceptor to form a colored charge-transfer product in the damaged area.
Another strategy reported for damage detection is to store color-changing indicators in isolated capsules or hollow fibers in a polymer matrix. However, these systems are limited by lack of a turn-on mechanism (e.g., the indicator is always “on”, fluorescent or colored), low contrast between the indicated region and the intact coating, and poor stability. One reported indication system utilizes two different types of capsules, one type containing crystal violet lactone leuco dye and the second type containing methyl-4-hydroxybenzoate color developer, embedded in a polymer coating with a solid silica gel color developer. This three-component system produces very low contrast color indication for a significant amount of indentation to the coating. The stability and controls (e.g., false positives) for this system is unknown.
Damage detection in coatings and composites is challenging. The damage-sensing smart coatings described above suffer from significant chemical and mechanical limitations, which make them less desirable to use in many situations. Some of the approaches require human or mechanical intervention, additional components (e.g., color developer or catalyst), activation (e.g., UV light) associated with significant external energy, and possess limited life-spans or are limited to modest temperatures. A large number of reported indicating systems employ catalysts or other specialty chemicals, which are often expensive, limited to narrow uses and provide less than ideal results in many situations. Many of these systems are unreliable and limited to special situations and conditions. A lot of these systems suffer from one or more of the following deficiencies: (i) poor color resolution, (iii) lack of versatility (e.g., unstable to certain matrices), (iii) unknown stability, (iv) modest responses and (iv) complicated or expensive processing.
Accordingly, there is a need for improved indicating systems of damage in coatings and composites. New material systems for coatings and composites that autonomously indicate the presence of damage and/or other environmental stresses prior to catastrophic failure of the coating or composite have the potential to decrease costs and enable more reliable operation in the field. In this patent, we describe a novel system to indicate damage in a material. The system is autonomic, self-powered, stable and adaptable to work on a wide variety of coating materials under different environmental conditions.